The fields radiated from an electronic equipment, which will be named afterwards as “AUT” an abbreviation for “Apparatus Under Test”, can be separated into radiation that is intentional, sought and inherent to its operation, and into non-intentional radiation.
Thus, a mobile phone transmits an expected radiation, required for communication, as well as complementary spurious emissions, which are non-intentional. A table-top computer, and in particular its screen, usually emits non-intentional radiation.
Non-intentional radiation can be divided between useful emissions, that is to say carriers of significant information, and non-useful emissions (noise).
At a time when telecommunications systems are enjoying extraordinary expansion, better understanding of emitted radiation is primordial since it is essential to guarantee a certain quality of service and better security of information.
Thus, as an example, the precise measurement of these different signals is indispensable for various applications, in particular:                forecast of radioelectric coverage;        verification of Electromagnetic Compatibility (EMC) and compliance with standards;        hardening and security, that is to say in particular the control and reduction of useful signals emitted;        analysis of the interaction of electromagnetic waves with persons.        
The measurements of these different signals are extremely difficult to implement by using present measurement techniques. As shown below, these measurements are even more difficult since the supply to the equipment concerned is not controlled.
Several techniques for measurements of radiation emitted by apparatus are known.
In order to limit emissions of non-intentional signals, competent international standards organizations for EMC, such as the CISPR (International Special Committee on Radio Interference) or the ANSI (American National Standards Institute), have laid out recommendations and set in place maximum non-intentional radiation levels for an AUT in a given configuration, from 30 MHz to 1 GHz. The measurement methods for a parasitic field radiated by an AUT are specified in the normative document of the CISPR 16-1, 1993 “Specifications of methods and measuring apparatus for radio-electric disturbances and immunity from radio-electric disturbances”, and also in the ANSI document C63.4-1988 (American national standard: “Methods of measurements of radio noise emissions from low voltage electrical and electronic equipment in the range of 10 kHz to 1 GHz”. These documents specify measurements in free space above a perfectly conducting plane.
The measurement protocol recommended by the CISPR has serious disadvantages:                at low frequency, the hypotheses of distant field used are a source of errors;        the measurement antenna is in the Fresnel zone (near field) of the AUT and the structure of the radiated waves is not a plane wave structure contrary to the approximation applied;        coupling phenomena difficult to take into account, and capable of affecting the validity of the measurement, may occur between the AUT and the measurement antenna, or between the antenna and its ground image, and bring about a modification of the value of the antenna factor.        because of the angular truncation above and the influence of the ground, the method does not make it possible to provide a precise diagram of the AUT radiation, nor to describe the emitted signals.        finally, these measurements are specified on normalized measurement sites which can be located in open areas (Open Area Test Site) or semi-echo-free chambers. These sites must be very large sized, are often costly and impractical, and are subject to many imperfections: spurious emissions, disturbing atmospheric phenomena, or furthermore poor performance of absorbents, shielding defects, etc.        
Near field (NF) measurement techniques have also been developed relying on the principle that the AUT supply is controlled (see J. Ch. Bolomey et al. “Spherical near-field facility for microwave coupling assessment in the 100 MHz-4 GHz frequency range)” in IEEE Trans. Antennas Propagat. vol. 40, pages 225-234, August 1998). According to this technique, measurements were taken by replacing the supply of the apparatus under study by a controlled supply. In this case, the field irradiated by the modified AUT can be measured at near distance in amplitude and in phase. This measurement is carried out by correlating the supply signal with the signal measured at a point. The field at any point in space can thus be calculated at any distance by using prior art techniques for NF (near field)—FF (far field) transformation.
However, it is clear that in this measurement configuration, the apparatus is not working under its normal utilization conditions, since its supply remains controlled. The validity of this type of measurement is, thus, limited to the measurement conditions (see J. Hald et al. “Spherical near field measurements”) Peter Peregrinus, Great Britain, 1998).
It certainly remains possible to carry out measurements without being in control of the source, but then the correlation operation mentioned above cannot be carried out and the near field measurements are in amplitude only. The near field distribution is thus known at a precise measurement distance which does not make it possible to determine the total emitted power. For this, one still has to measure the field in amplitude over a surface surrounding the AUT and located in a field far from the latter. These measurements are difficult and expensive to implement.
Such a method for measuring the near field radiated by an AUT whose supply is not under control has been described by A. Roczniac et al., “3-D Electromagnetic Field Modeling based on Near Field Measurements” in IEEE Conf. on Instruments and measurements, Brussels, Belgium, 1996. This method consists of measuring the electric field in amplitude only on two concentric hemispheres. By using a minimization algorithm, the phase is recalculated on one of the two hemispheres. However, according to the authors, this method has not been able to provide reliable limits for radiated electric field at every distance from the AUT.
Finally, a method for processing the data measured, called the FBAB method below for simplification—named after the initials of the inventors—has been proposed by the inventors of the present patent application in “Extension of techniques for near field measurement to the characterization of complex radiation systems in random state”, in National Microwave Days, Arcachon, France, May 1999. In this theoretical briefing, included here for reference, it is implicitly admitted, evidently in fictive fashion, that the signals are measured simultaneously at all the measuring points over the surface under consideration. It is theoretically possible to implement this processing method to know all the useful values mentioned above. But this hypothesis of simultaneous measurement at all points of the surface is not realistic because the complexity of the measuring apparatus needed would rule it out.
Finally, techniques are known, developed in reverberation chambers, with the following principle: the AUT is placed in a chamber with variable geometry over time, and with perfectly conductive walls. Then it is shown that, under certain conditions, one can measure at any point in the cavity the total average power radiated by the AUT (see J. Page “Stirred mode reverberation chambers for EMC measurements and radio type approvals” IEE 1998). Nonetheless this method of measurement is still not completely mastered and strict precautions have to be taken during the measurement of highly dispersive systems. Because of multiple reflections, the measurement of the radiation diagram of the AUT as well as the analysis of the shape of the signals emitted cannot be envisaged.
As far as present techniques for hardening measurement are concerned, that is to say techniques aimed at analyzing the “useful” signals emitted by an apparatus, that is the non-intentional signals containing information, it has been assumed until now that these signals are known a priori and it is verified that their level remains lower than threshold levels. No method for hardening measurement has been specified at present. Furthermore, present measurements do not make it possible to provide a radiation diagram associated with a useful signal, nor to know the total radiated power associated with this signal. In this sense, hardening measurements carried out at present by prior art methods remain very incomplete.
The aim of the present invention is to overcome these different disadvantages and limits of prior art.
More precisely, a first aim of the invention is to provide a method for near field measurement making it possible to measure, in normal utilization conditions, and with precision, all its radiation characteristics, including the radiated power, the diagram of radiation at any distance, as well as the shape of signals emitted associated with the radiation diagram of each of these signals.
A further aim of the invention is to develop a measurement technique capable of using the theoretical method developed by the same inventors, mentioned above, in a device of reduced complexity which nonetheless makes it possible to acquire the necessary data.
Another aim of the invention is to provide such a measurement method which can be exploited with compact measuring bases, of small dimensions.
A final aim of the invention is to provide a device for implementing such a procedure.